In a fluid driving device, such as a blower, a motor connects with an impeller to drive the impeller to rotation during operation. At startup of a single phase motor, the startup torque of the motor is small and fluctuates greatly. However, the impeller is stationary at its initial state, which requires the motor to have a large rotational inertia and startup load torque. As a result, vibrations may easily occur during startup of the motor; or even worse, motor startup failure may occur.
In a typical method of starting the single phase motor under load, a friction startup device is used to firstly drive the motor to rotate, which in turn progressively drives the impeller to rotate. Currently, the friction startup device consists of arcuate plates and an annular spring. Multiple arcuate plates are disposed on the impeller and are located on the same circle. The annular spring surrounds outer sides of the multiple arcuate plates. An end portion of a rotary shaft of the motor extends into a hole cooperatively defined by the multiple arcuate plates. As the rotary shaft of the motor rotates, the annular spring applies a constraint force to the multiple arcuate plates so that a friction force is generated between the arcuate plates and the rotary shaft. However, the friction force generated in this construction changes little with the change of the rotational speed, which is adverse to reducing of the rotational inertia and the startup load at the beginning of the startup and hence cannot effectively address vibrations and startup failure during the motor startup.
Therefore, it is urgently desired to reduce the rotational inertia and startup load applied to the rotary shaft during the motor startup to reduce the damage caused by the motor vibrations and startup failure.